Temperature-Dependent Kinetics of the Methyl Vinyl Ketone Oxide Criegee Intermediate: Self-Reaction and Reaction with...

Direct kinetic measurements are reported for the self-reaction and unimolecular decay of methyl vinyl ketone oxide (MVKOO), and its reactions with trifluoroacetic acid (TFA) and formic acid (FA). The syn-MVKOO stabilized Criegee intermediate was generated by laser flash photolysis of 1,3-diiodobut-2-ene in excess O₂ and monitored via its absorption at 360 nm using cavity ring-down spectroscopy. Time-resolved MVKOO decay traces recorded in the absence of added coreactants were analyzed to separate the first-order unimolecular thermal decomposition from second-order self-reaction contributions. Within the 270-330 K temperature and 40-200 Torr pressure ranges studied, MVKOO undergoes rapid second-order self-reaction with a rate coefficient k_SR = (12 ± 4) × 10^-10 cm³ s^-1 that shows no significant temperature or pressure dependence. At 292 K, the unimolecular decay rate coefficient is k_uni = (50  ±  21) s^-1, averaged over 40-200 Torr measurements, and shows a positive temperature dependence. Bimolecular reactions of MVKOO with FA and TFA were investigated under pseudo-first-order conditions, yielding rate coefficients of k_FA = (1.9 ± 0.2) × 10^-10 cm³ s^-1 and k_TFA = (3.8 ± 0.3) × 10^-10 cm³ s^-1 at 292 K, respectively. The MVKOO + TFA reaction exhibits a weak negative temperature dependence. The measured rate coefficients for MVKOO reaction with FA and TFA are consistent with the predictions from a structure-activity relationship based on dipole-mediated interactions. These findings quantify key unimolecular and bimolecular loss processes of MVKOO and further highlight the potential atmospheric significance of Criegee intermediate-acid chemistry in organic oxidation pathways.